Much like this report,
loopback-mounted ext4 sees hole-filling (on rawhide, but not F15)
http://thread.gmane.org/gmane.comp.file-systems.ext4/24454
but not requiring that one remove the backing file, this script shows the
same sort of corruption with a loopback-ext4-in-loopback-ext4 file system:
[Note these FS are small enough that they get a blocksize of 1024 by default.
When I specify -b 2048 or 4096, the problem goes away. ]
=============================================
#!/bin/sh
set -e
dd if=/dev/zero of=blob bs=9k count=1000 >/dev/null 2>&1
mkdir mnt
mkfs -q -t ext4 -F blob
mount -oloop blob mnt
c1=$PWD
cd mnt
dd if=/dev/zero of=blob bs=4k count=1000 >/dev/null 2>&1
mkdir m2
mkfs -q -t ext4 -F blob
mount -oloop blob m2
c2=$PWD
cd m2
# Create a reference file. Just like the following one,
# but with explicit NULs in place of holes.
perl -e '$n=1024; for (1..71) { print "\0"x$n, chr($_)x$n };' \
-e 'close *STDOUT or die "$!"' > ref
# Seek 1KB, write 1KB of data, seek 1KB, write 1KB of data, etc....
perl -e '$n = 1 * 1024; *F = *STDOUT;' \
-e 'for (1..71) { sysseek (*F, $n, 1)' \
-e '&& syswrite (*F, chr($_)x$n) or die "$!"}' > j1
# filefrag -vs j1
sync
cmp -s ref j1 && fail=0 || fail=1
cd /
umount "$c2/m2" "$c1/mnt"
rm -rf "$c1/blob" "$c1/mnt"
exit $fail
=============================================
This shows that it fails most of the time for me in tmpfs on a rawhide
guest (2.6.39-0.rc3.git2.0.fc16.x86_64) running on an F15 host. YMWV.
$ while :; do ./ext4-bug; printf $?; done
111111110111111110111111011111111110101111111111
[Exit 130 (INT)]
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